1. Field of the Invention
This invention relates generally to methods and apparatus for monitoring electro-conductive fluids, and more specifically relates to electronic circuits and methods for monitoring electro-conductive fluids in process control and the electrolyte in batteries to determine the state of charge of the batteries.
2. Description of the Prior Art
There is a continuing need for accurate instruments capable of monitoring the state of charge of batteries, especially for batteries used in electric vehicles. The conventional approach used in monitoring starter batteries in engine driven vehicles cannot yield the accuracy required to inform the operator of the state of charge of batteries, for battery terminal voltages are not representative of battery charge until the battery has rested for about twenty four hours after being charged or discharged.
The common method of ascertaining battery health by using a hygrometer to determine the specific gravity of the electrolyte is not readily useful for continuous monitoring. A motorist concerned about reaching his destination in an electric car will not want to stop periodically to employ a hygrometer. Further, were a method of continuous measurement of specific gravity to be found, the specific gravity readings would only furnish a useful approximation of remaining charge within a narrow range of temperatures close to the standard of 25 degrees Celsius, even when temperature compensation of the hygrometer readings was applied.
Lead-acid battery specifications typically state the number of engine cranking amperes available for a specified period in minutes, at warm and cold temperatures, and reveal that cranking power available at cold temperatures is only a fraction of that available at warm temperatures for the same fully charged battery. The motorist attempting to start his engine in cold weather may only have the ability to obtain 40 percent of warm weather starting current, yet his hygrometer will show a very high specific gravity.
Conventional battery chargers are controlled by time and/or voltage considerations; no direct measure of actual charge is made. The best chargers approximate the Peukert charging algorithm, charging commonly in three, or twelve, steps by controlling charging current and voltage, within certain time constraints. Yet even these continual chargers do not directly measure actual electrolyte condition.
In laboratories, in an attempt to ascertain state of charge, Coulomb cells are occasionally employed. In these cells, a portion of battery charge and discharge currents is routed through the cell, which deposits or removes silver along the glass walls of the cell, thus providing a visual approximation of net charge. However, this instrument ignores the loss in charge caused by recharge inefficiency. One electronic system designed for marine use employs this same general approach. It integrates charge and discharge currents over time, applying a general recharge efficiency correction factor to the recharge current to yield a better approximation in remaining ampere-hours. Again, no direct measurement of actual recharge attained is attempted with this system. Other long-established analytical techniques common to laboratories, such as spectral analysis, are not well-suited to continuous monitoring of battery acid or other electrolytic fluids.
Battery power available at any moment is directly proportional to the concentration of ions in battery electrolyte at that moment. Direct measurement of ion concentration by direct, electrical means yields the best measure of battery capability at that moment. Battery electrolytes, by their very nature, must conduct electrical currents, including those currents superimposed by test instrumentation. Thus, under the action of an impressed test voltage, the resulting flow of ions and electrons is readily measured and is directly related to the density of the available charge carriers, i.e., the electrons and ions.
Howes, in U.S. Pat. No. 4,129,824, teaches the measurement of cell conductivity in a lead-acid battery using an alternating current sinusoidal waveform impressed upon two electrodes immersed in the battery electrolyte. The resulting current is sent through a fixed resistance. The voltage across that resistance is rectified by a diode, filtered to provide a steady voltage and compared to a direct current reference voltage by a differential amplifier. Howes teaches the use of a thermistor temperature sensor in his measuring probe in order to compensate for the thermal change in cell resistivity due to temperature by using a portion of the thermistor's voltage as part of the reference voltage divider string. The resulting output voltage is expressed as specific gravity. Any vehicle operator attempting to relate the output of Howes' apparatus to charge remaining on the battery will be only vaguely informed of the remaining life of his charge, the distance the vehicle can travel, and the like, especially in cold temperatures. For more precise information, he must refer to battery performance tables and perform complex calculations better left to a microprocessor.
In electrolytes, the ions which are propelled from electrode to electrode by the impressed voltage must move through a viscous medium, for example, through the dilute sulfuric acid solution in lead-acid batteries. As mentioned previously, Howes teaches in U.S. Pat. No. 4,129,824 the use of an alternating current sinusoidal waveform in his battery monitor. Howes uses a half-wave rectification approach. This requires heavy filtering and results in long delays in generating signals used for monitoring the state of charge on the battery. In addition, with sine wave ion propulsion, as taught by Howes, the varying electromotive force may have varying effects on the various components of ion mobility limitations, and steady-state ion flow is not attained during each half cycle.
There is also a continuing need for accurate instruments capable of monitoring electro-conductive fluids in process control. The control of salinity in brine solution--in refrigeration, pickling of vegetables, and in salt water aquariums--is one example. Another example is the control of acid concentration in electroplating tanks or for filling lead-acid batteries. Other examples include control of dilutions and mixtures of acids and alkalis and other chemical compounds. In beverages, both for carbonated and non-carbonated soft drinks and flavored waters, accurate control of the sugar content is required, not only for cost considerations but also for taste and quality control. Being electrolytes, all of these fluids are also well suited to direct electrical measurement of their ionic concentrations.
Modern instrumentation and process control technology relies heavily on digital processing. Thus, it is highly desirable that process control monitoring instruments be highly compatible with digital processing equipment. Analog devices are susceptible to thermally-induced changes in gain; direct-current analog devices are also susceptible to drifts in offset voltages or currents, zero levels, and the like. Hence, there is a need for monitoring instruments which are compatible with microprocessors, and which employ digital techniques to the maximum practical extent.